Pure titanium and titanium alloy (below, simply referred to as “titanium”) exhibit remarkably superior corrosion resistance in atmospheric environments, so are being used for building material applications such as roofs and walls at seaside areas. Approximately 10 years has elapsed since titanium began to be used for roofing materials and the like, but up until now there have been no examples where corrosion has been reported as occurring. However, depending on the usage environment, there are cases when the titanium surface used over an extended period of time changes to a dark gold color.
The discoloration is limited to the uppermost surface layer, so does not impair the anticorrosive function of the titanium, but from the viewpoint of aesthetic appearance, this sometimes becomes a problem. To eliminate the discoloration, the titanium surface is wiped using nitric acid, hydrofluoric acid, and other acid or the discolored part is removed by light polishing using abrasive papers and abrasive agents. When treating a large area titanium surfaces such as a roof, there are problems from the viewpoint of workability.
The cause of the discoloration in titanium still has not been sufficiently explained, but occurrence due to Fe, C, SiO2, and the like suspending in the atmosphere adhering to the titanium surface and the possibility of occurrence due to an increase in the film thickness of the titanium oxide on the titanium surface are suggested. Further, as a method to reduce the discoloration, as disclosed in Japanese Patent Publication (A) No. 2000-1729, applying titanium having an oxide film of 10 nm or less on the titanium surface and given a surface carbon concentration of 30 at (atomic) % or less has been reported to being effective.
However, the present inventors carefully studied the effect of the thickness of the oxide film and carbon concentration on the surface on the discoloration using surface analysis of roofing materials made of titanium causing discoloration at different areas of Japan so as to prevent discoloration and as a result discovered that, unlike Japanese Patent Publication (A) No. 2000-1729, an oxide film thickness which is relatively thick is effective for the improvement of the resistance to discoloration. Further, in regards to the carbon, they discovered that discoloration is promoted by the carbon concentrated on the surface producing carbides.
As a result, they proposed titanium which has a relatively thick oxide film thickness and a low carbon concentration on the surface (142nd ISIJ Meeting, Current Advances in Materials and Processes, CAMP-ISIJ Vol. 14 (2001)-1336, 1337, 1338, and 1339). Further, by thickening the oxide film on the titanium surface, it is possible to reduce the carbon concentration at the titanium surface and form a titanium oxide layer as described above to greatly improve the resistance to discoloration even for colored titanium using the interaction. However, in harsh acid rain environments, the titanium oxide layer is altered in some cases, so colored titanium having a further superior resistance to discoloration is being sought.